Coupled ring oscillator and method for laying out the same

ABSTRACT

A coupled ring oscillator includes n ring oscillators ( 20 ) each including m inverter circuits ( 10 ), and a phase-coupling loop ( 40 ) in which m×n phase-coupling circuits ( 30 ), each of which couples signal phases at two points in a certain phase mode, are connected with each other to form a loop. Connection points at which the inverter circuits ( 10 ) are connected with each other and the connection points at which the phase-coupling circuits ( 30 ) are connected with each other are connected bijectively; and each of the inverter circuits ( 10 ) is connected between two points that divide the phase-coupling circuits ( 30 ) into two parts at a certain ratio.

RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/JP2006/310955, filed on May 25, 2006,which in turn claims the benefit of Japanese Application No.2005-155371, filed on May 27, 2005 and Japanese Application No.2005-268019, filed on Sep. 15, 2005, the disclosures of whichApplications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to ring oscillators and a method forlaying out the ring oscillators, and more particularly relates to acoupled ring oscillator which uses a plurality of ring oscillators andis capable of generating highly accurate phase information and a methodfor laying out the coupled ring oscillator.

BACKGROUND ART

In order to record information on optical disc media such as a DVD(digital versatile disc), it is necessary to generate a specialwrite-operation waveform for suppressing interference of a write signal.Generation of such a special write-operation waveform requires highlyaccurate phase information that is finer than one-fortieth of the writedata rate. However, it is difficult to realize such extremely fine phaseaccuracy by a single inverter chain (a ring oscillator), because thephase delay thereof is shorter than a delay of a single invertercircuit. Therefore, conventionally, a plurality of ring oscillators areused and the respective inverter circuits in the ring oscillators areconnected by phase-coupling circuits so as to slightly change the outputphase of each ring oscillator, whereby phase information finer than thatthat can be generated by a single ring oscillator is produced (see, forexample, Patent Documents 1, 2, and 3).

Also, a resistor ring, in which a plurality of resistors are connected,is provided, and the connection points of the resistors in the resistorring are connected with the connection points of phase delay elements ina plurality of ring oscillators are connected to thereby produce finephase information (see Patent Document 4, for example)

-   Patent Document 1: Specification of Patent No. 550030-   Patent Document 2: Specification of U.S. Pat. No. 5,475,344-   Patent Document 3: Specification of U.S. Pat. No. 5,717,362-   Patent Document 4: Specification of US patent application    publication No. 2006/0049879

DISCLOSURE OF THE INVENTION

However, in the conventional layout in which the respective invertercircuits in the ring oscillators are connected by the phase-couplingcircuits, the wire length of some of the signal wires is extremelyincreased and only some of the inverter circuits are thus under heavyload. Such layout errors cause variations in the driving timing of theinverter circuits, making it difficult to generate highly accurate phaseinformation.

In view of the above problem, it is therefore an object of the presentinvention to provide a coupled ring oscillator including a plurality ofring oscillators and capable of generating highly accurate phaseinformation. Another object of the present invention is to provide asimple method for laying out the coupled ring oscillator.

In order to solve the above problem, an inventive coupled ringoscillator includes: n ring oscillators each including m invertercircuits; and a phase-coupling loop in which m×n phase-couplingcircuits, each of which couples signal phases at two points in a certainphase mode, are connected with each other to form a loop, whereinconnection points at which the inverter circuits are connected with eachother and the connection points at which the phase-coupling circuits areconnected with each other are connected bijectively; and each of theinverter circuits is connected between two points that divide thephase-coupling circuits into two parts at a certain ratio.

Then, the connection points at which the m inverter circuits in each ofthe n ring oscillators are connected with each other and the connectionpoints at which the m×n phase-coupling circuits are connected with eachother are connected bijectively, and each of the inverter circuits isconnected between two points that divide the m×n phase-coupling circuitsin the phase-coupling loop into two parts at a certain ratio. Therefore,all of the inverter circuits have substantially the same wire length,allowing highly accurate and fine phase information to be generated.

Specifically, the phase-coupling circuits each couple signal phases attwo points in a reverse phase mode; and each of the inverter circuits isconnected between two points that divide the phase-coupling circuitsinto n phase-coupling circuits and n×(m−1) phase-coupling circuits.

Also, specifically, the phase-coupling circuits each couple signalphases at two points in a common mode; and each of the inverter circuitsis connected between two points that divide the phase-coupling circuitsinto n×(m−1)/2 phase-coupling circuits and n×(m+1)/2 phase-couplingcircuits.

An inventive method for laying out a coupled ring oscillator includesthe steps of: (a) connecting m×n basic cells each including aphase-coupling element and a circuit element, wherein the phase-couplingelement corresponds to two connected half-circuits, each equivalent tohalf of a phase-coupling circuit for coupling two signal phases at twopoints in a reverse phase mode, at both ends of the phase-couplingcircuit, the circuit element includes an inverter circuit in whicheither an input terminal or an output terminal is connected to aconnection point at which the two half-circuits are connected with eachother, and the m×n basic cells are connected into a loop so that eachphase-coupling circuit is formed between adjacent ones of thephase-coupling elements; and (b) connecting, in any two of the basiccells existing (n−1) apart, the input terminal of the inverter circuitin one of the two basic cells with the output terminal of the invertercircuit in the other basic cell.

Then, the m×n basic cells each including the phase-coupling element andthe inverter circuit are appropriately laid out to form a loop, wherebythe above-mentioned coupled ring oscillator is obtained that includesthe n ring oscillators, in each of which the m inverter circuits areconnected with each other, and the phase-coupling loop, in which the m×nphase-coupling circuits are connected into a loop.

Another inventive method for laying out a coupled ring oscillatorincludes the steps of: (a) connecting m×n basic cells each including aphase-coupling circuit for coupling two signal phases at two points in acertain phase mode and a circuit element, wherein the circuit elementincludes an inverter circuit in which either an input terminal or anoutput terminal is connected to one end of the phase-coupling circuit,and the m×n basic cells are connected into a loop so that thephase-coupling circuits are connected with each other; and (b)connecting, in any two of the basic cells existing (n−1) apart, theinput terminal of the inverter circuit in one of the two basic cellswith the output terminal of the inverter circuit in the other basiccell.

Specifically, each of the phase-coupling circuits couples signal phasesat two points in a reverse phase mode; and in the step (a), the basiccells are connected so that adjacent ones of the phase-coupling circuitsare connected with each other.

Also, specifically, each of the phase-coupling circuits couples signalphases at two points in a common mode; and in the step (a), the basiccells are connected so that alternate ones of the phase-couplingcircuits are connected with each other.

Then, the m×n basic cells each including the phase-coupling circuit andthe inverter circuit are appropriately laid out to form a loop, wherebythe above-mentioned coupled ring oscillator is obtained that includesthe n ring oscillators, in each of which the m inverter circuits areconnected with each other, and the phase-coupling loop, in which the m×nphase-coupling circuits are connected into a loop.

As described above, the inventive coupled ring oscillator generateshighly accurate and fine phase information. Also, the inventive coupledring oscillator is easily formed by connecting the basic cells into aloop.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the configuration of a coupled ring oscillator according toa first embodiment.

FIGS. 2A and 2B show circuit configurations of phase-coupling circuitsshown in FIG. 1.

FIG. 3 shows the configuration of a coupled ring oscillator according toa second embodiment.

FIGS. 4A, 4B, 4C, and 4D show circuit configurations of phase-couplingcircuits shown in FIG. 3.

FIGS. 5A, 5B, 5C, and 5D show circuit configurations of basic cells inthe coupled ring oscillator according to the first embodiment, in whichinverter circuits are included.

FIGS. 6A, 6B, 6C, and 6D show circuit configurations of basic cells inthe coupled ring oscillator according to the first embodiment, in whichNMOS transistors are used.

FIG. 7 shows an exemplary layout of the basic cell shown in FIG. 5.

FIG. 8 shows relation between phase-coupling elements and aphase-coupling circuit when the phase-coupling elements are composed ofinverter circuits.

FIGS. 9A, 9B, 9C, and 9D show circuit configurations of basic cells inthe coupled ring oscillator according to the second embodiment.

FIGS. 10, 11, 12, 13, 14, and 15 show exemplary configurations of acoupled ring oscillator of a three-stage, three-set type.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings.

First Embodiment

FIG. 1 shows the configuration of a coupled ring oscillator according toa first embodiment. The coupled ring oscillator shown in FIG. 1 includesthree (n=3) ring oscillators 20, each composed of five (m=5) invertercircuits 10, and a phase-coupling loop 40 in which fifteen (m×n=15)phase-coupling circuits 30 are connected to form a loop.

The three ring oscillators 20 and the phase-coupling loop 40 are laidout in a nested pattern, in which the phase-coupling loop 40 isinnermost. The connection points at which the phase-coupling circuits 30are connected with each other and the connection points at which theinverter circuits 10 are connected with each other are connected. Thatis, the connection points of the phase-coupling circuits 30 and theconnection points of the inverter circuits 10 are connected bijectively.Furthermore, each inverter circuit 10 is parallel-connected with threeconnected phase-coupling circuits 30. In other words, each invertercircuit 10 is connected between two points that divide the fifteen(m×n=15) phase-coupling circuits 30 into three (n=3) and twelve(n×(m−1)=12).

FIGS. 2A and 2B show exemplary circuit configurations of thephase-coupling circuits 30 shown in FIG. 1. Each phase-coupling circuit30 of this embodiment couples signal phases at two points in a reversephase mode. When the phase-coupling circuit 30 is composed of invertercircuits, the phase-coupling circuit 30 takes the form of a latchcircuit in which the input terminal of one of the two inverter circuits31 is connected to the output terminal of the other, as shown in FIG.2A. When the phase-coupling circuit 30 is composed of MOS transistors,for example, NMOS transistors, the phase-coupling circuit 30 takes theform of a latch circuit in which the gate of one of the two NMOStransistors 32 is connected to the drain of the other, as shown in FIG.2B.

The circuit layout of the coupled ring oscillator shown in FIG. 1 iscompletely different from the conventional one, and all of the invertercircuits 10 have substantially the same wire length. In the coupled ringoscillator of this embodiment, the phenomenon in which only some of theinverter circuits are under heavy load does not occur, which enableshighly accurate and fine phase information to be generated. Also, eachphase information can be taken out from each connection point of thephase-coupling circuits 30 in the phase-coupling loop 40, such that thephase information can be taken very easily.

Second Embodiment

FIG. 3 shows the configuration of a coupled ring oscillator according toa second embodiment. As in the first embodiment, the coupled ringoscillator shown in FIG. 3 includes n(n=3) ring oscillators 20, eachcomposed of five (m=5) inverter circuits 10, and a phase-coupling loop40 in which fifteen (m×n=15) phase-coupling circuits 30 are connected toform a loop.

As in the first embodiment, the three ring oscillators 20 and thephase-coupling loop 40 are laid out in a nested pattern in which thephase-coupling loop 40 is innermost. The connection points of thephase-coupling circuits 30 and the connection points of the invertercircuits 10 are connected bijectively.

Unlike in the first embodiment, the phase-coupling circuits 30 of thisembodiment each couple signal phases at two points in a common mode.That is, the phase-coupling circuits 30 are connected in such a mannerthat each phase-coupling circuit 30 connects two of the connectionpoints of the inverter circuits 10 in two ring oscillators 20 thatshould be in phase with each other. Therefore, the phase-coupling loop40 forms two turns and is closed, while connecting the ring oscillators20 in an alternate manner. In other words, each inverter circuit 10 isconnected between two points that divide the fifteen phase-couplingcircuits 30 into six (n×(m−1)/2=6) and nine (n×(m+1)/2=9).

The phase-coupling circuits 30 of this embodiment each couple signalphases at two points in a common mode, and thus can be realized byresistive elements. Exemplary configurations of the phase-couplingcircuits 30 will be described below.

FIG. 4A shows an example in which the phase-coupling circuit 30 iscomposed of an NMOS transistor 33, in which a voltage Vg1 that is higherthan a threshold voltage Vth1 is applied between the gate and the sourceor between the gate and the drain. Preferably, a control voltage of theoscillation frequency of the ring oscillators 20 is applied to the gateof the NMOS transistor 33. Then, when the control voltage rises, forexample, gm (transconductance) of the NMOS transistor 33 increases withthe oscillation frequency and the amplitude of the ring oscillators 20,whereby the strength of the common mode coupling is increased.Therefore, the linearity of signal phase interpolation by thephase-coupling circuit 30 is maintained in a wide band.

FIG. 4B shows an example in which the phase-coupling circuit 30 iscomposed of a PMOS transistor 34, in which a voltage Vg2 that is lowerthan a threshold voltage Vth2 is applied between the gate and the sourceor between the gate and the drain. Preferably, the gate of the PMOStransistor 34 is grounded. Then, when the control voltage of theoscillation frequency of the ring oscillators 20 rises, for example, gm(transconductance) of the PMOS transistor 34 increases with theoscillation frequency and the amplitude of the ring oscillators 20,whereby the strength of the common mode coupling is increased.Therefore, the linearity of signal phase interpolation by thephase-coupling circuit 30 is maintained in a wide band.

FIG. 4C shows an example in which the phase-coupling circuit 30 iscomposed of a transfer gate 35 in which the NMOS transistor 33 and thePMOS transistor 34 are combined. The combination of the NMOS and PMOStransistors increases the effect of linearity further.

FIG. 4D shows an example in which the phase-coupling circuit 30 iscomposed of a resistance element 36. Although the resistance element 36has the highest linearity, the resistance value does not change with theoscillation frequency of the ring oscillators 20. Therefore, theresistive element 36 is not so effective in signal phase interpolation.

As described above, in this embodiment, since the phases of the ringoscillators 20 are coupled by the resistive elements having highlinearity, signal phase interpolation is performed more accurately.

It should be noted that the phase-coupling loop 40 is not limited to theconfiguration in which the phase-coupling loop 40 forms two turns and isclosed. However, if the phase-coupling loop 40 is formed according tothe below-described layout method, the phase-coupling loop 40 has theconfiguration shown in FIG. 3.

In the coupled ring oscillator according to the present invention, thenumber of ring oscillators 20 is not limited to three and the number ofinverter circuits 10 forming each ring oscillators 20 is not limited tofive.

(Layout Method)

Next, a method for laying out the coupled ring oscillators helpful forunderstanding the present invention will be described. The coupled ringoscillators according to the preferred embodiments have a geometricalregularity as shown in FIGS. 1 and 3 and thus can be easily formed bylaying out a plurality of “basic cells”.

FIGS. 5A-5D and 6A-6D show exemplary circuit configurations of basiccells in the coupled ring oscillator of the first embodiment. In FIGS.5A-5D, the basic cells are formed using inverter circuits, while inFIGS. 6A-6D, the basic cells are formed using NMOS transistors. Thebasic cells shown in FIGS. 5A and 5B and FIGS. 6A and 6B are eachcomposed of an inverter circuit 10, which is part of a ring oscillator20, and a phase-coupling element 30′. The phase-coupling element 30′ isequivalent to a configuration in which two half-circuits, eachcorresponding to half of a phase-coupling circuit 30 having asymmetrical circuit configuration, are connected at both ends of thephase-coupling circuit 30. For example, the phase-coupling elements 30′shown in FIGS. 5A and 5B have a circuit configuration in which the inputterminals of two inverter circuits 31 are connected with each other.This corresponds to a configuration in which two inverter circuits 31 ashalf-circuits of the phase-coupling circuit 30 shown in FIG. 2A areconnected at both ends of the phase-coupling circuit 30. Also, thephase-coupling elements 30′ shown in FIGS. 6A and 6B have a circuitconfiguration in which the gates of two NMOS transistors 32 areconnected with each other. This corresponds to a configuration in whichtwo NMOS transistors 32 as half-circuits of the phase-coupling circuit30 shown in FIG. 2B are connected at both ends of the phase-couplingcircuit 30.

The basic cells shown in FIGS. 5A and 6A have a configuration in whichthe input terminal of the inverter circuit 10 is connected with thephase-coupling element 30′. On the other hand, the basic cells shown inFIGS. 5B and 6B have a configuration in which the output terminal of theinverter circuit 10 is connected with the phase-coupling element 30′. Inthis way, either the input terminal or the output terminal of theinverter circuit 10 may be connected with the phase-coupling element30′. However, it is preferable that the input terminal be connected withthe phase-coupling element 30′. FIG. 7 shows an exemplary layout of thebasic cell shown in FIG. 5A. By connecting the input terminal of theinverter circuit 10 with the phase-coupling element 30′, the gates ofthe transistors forming the inverter circuit 10 are connected by acommon wire with the transistors forming the inverter circuits 31. Theconfiguration of the basic cell is thus simplified significantly. Thiseffect is also obtained in the basic cell shown in FIG. 6A.

On the other hand, FIG. 5C shows an exemplary configuration of a basiccell in which the input terminal of an inverter circuit 10 as part of aring oscillator 20 is connected with the phase-coupling circuit 30 shownin FIG. 2A. FIG. 5D shows an exemplary configuration of a basic cell inwhich the output terminal of an inverter circuit 10 as part of a ringoscillator 20 is connected with the phase-coupling circuit 30 shown inFIG. 2A. FIG. 6C shows an exemplary configuration of a basic cell inwhich the input terminal of an inverter circuit 10 as part of a ringoscillator 20 is connected with the phase-coupling circuit 30 shown inFIG. 2B. FIG. 6D shows an exemplary configuration of a basic cell inwhich the output terminal of an inverter circuit 10 as part of a ringoscillator 20 is connected with the phase-coupling circuit 30 shown inFIG. 2B. In this manner, the basic cell may be composed of a singleinverter circuit 10 and a single phase-coupling circuit 30.

The coupled ring oscillator of the first embodiment is formed in thefollowing manner. First, m×n basic cells are laid out appropriately soas to form a loop. The phase-coupling elements 30′ or the phase-couplingcircuits 30 in adjacent basic cells are connected with each other toform a single phase-coupling loop 40. The input terminal of the invertercircuit 10 in one of any two basic cells existing (n−1) apart isconnected with the output terminal of the inverter circuit 10 in theother basic cell, thereby forming m ring oscillators 20. If the layoutis performed with m=5 and n=3, the coupled ring oscillator of thefive-stage, three-set type shown in FIG. 1, for example, is obtained.

FIG. 8 shows the relation between the phase-coupling elements 30′ andthe phase-coupling circuit 30 when the phase-coupling elements 30′ arecomposed of inverter circuits. As shown in FIG. 8, when twophase-coupling elements 30′ are connected to each other, a singlephase-coupling circuit 30 is formed. More specifically, in the adjacentphase-coupling elements 30′, the input terminal of one inverter circuit31 in one of the two phase-coupling elements 30′ is connected with theoutput terminal of one inverter circuit 31 in the other phase-couplingelement 30′. Where the phase-coupling elements 30′ are composed of NMOStransistors, the gate of one NMOS transistor 32 in one of the adjacentphase-coupling elements 30′ is connected with the gate of one NMOSransistor 32 in the other phase-coupling element 30′. In this manner,m×n phase-coupling elements 30′ are connected to form a loop, wherebythe phase-coupling loop 40 is obtained in which the m×n phase-couplingcircuits 30 are connected into a loop.

FIGS. 9A-9D show exemplary circuit configurations of the basic cells inthe coupled ring oscillator according to the second embodiment. FIGS. 9Aand 9B show exemplary configurations of the basic cells in which theinput terminal of an inverter circuit 10 is connected with aphase-coupling circuit 30. FIGS. 9C and 9D show exemplary configurationsof the basic cells in which the output terminal of an inverter circuit10 is connected with a phase-coupling circuit 30.

The coupled ring oscillator of the second embodiment is formed in thefollowing manner. First, m×n basic cells are laid out appropriately soas to form a loop. The phase-coupling circuits 30 in adjacent basiccells are connected with each other to form a single phase-coupling loop40. Also, the input terminal of the inverter circuit 10 in one of anytwo basic cells existing (n−1) apart is connected with the outputterminal of the inverter circuit 10 in the other basic cell, therebyforming m ring oscillators 20. If the layout is performed with m=5 andn=3, the coupled ring oscillator of the five-stage, three-set type shownin FIG. 3, for example, is obtained.

(Exemplary Configurations)

Exemplary configurations of coupled ring oscillators of a three-stage,three-set type formed according to the above layout methods will beshown.

FIGS. 10, 11 and 12 show exemplary configurations of a coupled ringoscillator according to the first embodiment. In the exemplaryconfiguration shown in FIG. 10, nine basic cells 50 are divided intofour and five and the layout is performed so that the four basic cells50 and the five basic cells 50 face each other. In the exemplaryconfiguration shown in FIG. 11, the layout is performed so that thebasic cells 50 are located on four sides. In the exemplary configurationshown in FIG. 12, nine basic cells 50 are divided into four and five andthe layout is performed so that the four basic cells 50 and the fivebasic cells 50 face in the same direction. In each of the exemplaryconfigurations, the basic cell shown in FIG. 5A or 6A is used.

On the other hand, FIGS. 13, 14, and 15 show exemplary configurations ofa coupled ring oscillator of the second embodiment. The configurationsin FIGS. 13, 14, and 15 correspond to those shown in FIGS. 10, 11 and12, respectively. In each of the exemplary configurations shown in FIGS.13, 14, and 15, the basic cell shown in FIG. 9A is used.

As shown in FIGS. 12 and 15, in the coupled ring oscillators accordingto the preferred embodiments, the ring oscillators 20 and thephase-coupling loop 40 do not necessarily have to be arranged in anested pattern. In cases where the ring oscillators 20 and thephase-coupling loop 40 are laid out in a nested pattern, thephase-coupling loop 40 is placed innermost. This is because the numberof stages in which the phase-coupling circuits 30 are connected in thephase-coupling loop 40 is greater than the number of stages in which theinverter circuits 10 are connected in each ring oscillator 20, whichcauses more delays in the phase-coupling loop 40. However, the operationspeed of the phase-coupling loop 40 must be higher than that of the ringoscillators 20. Therefore, in order to reduce the delays as much aspossible, the phase-coupling loop 40 is preferably placed innermost inthe nest so that the wire length thereof is shortened.

INDUSTRIAL APPLICABILITY

The inventive coupled ring oscillator generates highly accurate and finephase information and is thus applicable, for example, to ahigh-resolution phase generating circuit such as a write strategycircuit for generating a write clock in an optical disk drive.

1. A coupled ring oscillator comprising: n ring oscillators eachincluding m inverter circuits and wires connecting the m invertercircuits arranged so that the m inverter circuits and the wiresconstitute n loop circuits each having a loop shape in a plan view; anda phase-coupling loop in which m×n phase-coupling circuits, each ofwhich couples signal phases at two points in a certain phase mode, areconnected with each other in a ring configuration in a plan view,wherein connection points at which the inverter circuits are connectedwith each other and the connection points at which the phase-couplingcircuits are connected with each other are connected bijectively, eachof the inverter circuits is connected between two points that divide thephase-coupling circuits into two parts at a certain ratio, and each ofthe n loop circuits of the ring oscillators does not include any of them×n phase-coupling circuits, and encloses at least one commonphase-coupling circuit in a plan view.
 2. The coupled ring oscillator ofclaim 1, wherein the phase-coupling circuits each couple signal phasesat two points in a reverse phase mode; and each of the inverter circuitsis connected between two points that divide the phase-coupling circuitsinto n phase-coupling circuits and n×(m−1) phase-coupling circuits. 3.The coupled ring oscillator of claim 2, wherein the phase-couplingcircuits each include two inverter circuits and an input terminal and anoutput terminal of one of the inverter circuits are connected with anoutput terminal and an input terminal of the other inverter circuit,respectively.
 4. The coupled ring oscillator of claim 2, wherein thephase-coupling circuits each include two MOS transistors of the samepolarity and a gate and a drain of one of the MOS transistors areconnected with a drain and a gate of the other MOS transistor,respectively.
 5. The coupled ring oscillator of claim 4, wherein the MOStransistors are NMOS transistors.
 6. The coupled ring oscillator ofclaim 1, wherein, when the inner most ring oscillator is a first ringoscillator and the outermost ring oscillator is an m-th ring oscillator,a loop circuit of a k-th ring oscillator encloses a loop circuit of a(k−1)-th ring oscillator in a plan view where k=2 to m, and a loopcircuit of the first ring oscillator encloses the m×n phase-couplingcircuits in a plan view.
 7. The coupled ring oscillator of claim 1,wherein each loop circuit of the n ring oscillators has a substantiallyrectangular shape in a plan view.
 8. A method for laying out a coupledring oscillator, comprising the steps of: (a) connecting m×n basic cellseach including a phase-coupling element and a circuit element, whereinthe phase-coupling element corresponds to two connected half-circuits,each equivalent to half of a phase-coupling circuit for coupling twosignal phases at two points in a reverse phase mode, at both ends of thephase-coupling circuit, the circuit element includes an inverter circuitin which either an input terminal or an output terminal of the invertercircuit is connected to a connection point at which the twohalf-circuits are connected with each other, and the m×n basic cells areconnected by wires so that inverter circuits and the wires constitute nloop circuits and each phase-coupling circuit is formed between adjacentones of the phase-coupling elements, wherein each of the n loop circuitsconstituted by the inverter circuits and the wires encloses thephase-coupling elements in a plan view; and (b) connecting, in any twoof the basic cells existing (n−1) apart, the input terminal of theinverter circuit in one of the two basic cells with the output terminalof the inverter circuit in the other basic cell.
 9. The method of claim8, wherein each of the phase-coupling elements includes two invertercircuits as the two half-circuits, and input terminals of the twoinverter circuits are connected with each other; and the method furtherincludes the step (c) of connecting, in adjacent two of thephase-coupling elements, the input terminal of one of the invertercircuits in one of the two phase-coupling elements with an outputterminal of one of the inverter circuits in the other phase-couplingelement.
 10. The method of claim 9, wherein the input terminals of thetwo inverter circuits in each phase-coupling element are connected withthe input terminal of the inverter circuit in each of the basic cells.11. The method of claim 8, wherein each of the phase-coupling elementsincludes two MOS transistors as the two half-circuits, and gates of thetwo MOS transistors are connected with each other; and the methodfurther includes the step (c) of connecting, in adjacent two of thephase-coupling elements, the gate of one of the MOS transistors in oneof the two phase-coupling elements with a drain of one of the MOStransistors in the other phase-coupling element.
 12. The method of claim11, wherein the gates of the two MOS transistors in each phase-couplingelement are connected with the input terminal of the inverter circuit ineach of the basic cells.
 13. The method of claim 11, wherein the MOStransistors are NMOS transistors.
 14. A method for laying out a coupledring oscillator, comprising the steps of: (a) connecting m×n basic cellseach including a phase-coupling circuit for coupling two signal phasesat two points in a certain phase mode and a circuit element, wherein thecircuit element includes an inverter circuit in which either an inputterminal or an output terminal of the inverter circuit is connected toone end of the phase-coupling circuit, and the m×n basic cells areconnected by wires so that inverter circuits and the wires constitute nloop circuits and the phase-coupling circuits are connected with eachother, wherein each of the n loop circuits constituted by the invertercircuits and the wires encloses the phase-coupling elements in a planview; and (b) connecting, in any two of the basic cells existing (n−1)apart, the input terminal of the inverter circuit in one of the twobasic cells with the output terminal of the inverter circuit in theother basic cell.
 15. The method of claim 14, wherein each of thephase-coupling circuits couples signal phases at two points in a reversephase mode; and in the step (a), the basic cells are connected so thatadjacent ones of the phase-coupling circuits are connected with eachother.
 16. The coupled ring oscillator of claim 1, wherein each of theloop circuit of the ring oscillators encloses all of the m×nphase-coupling circuits in a plan view.
 17. The method of claim 8,wherein the loop circuits constituted by the inverter circuits and thewires do not include any of the phase-coupling elements, and each of then loop circuits constituted by the inverter circuits and the wiresencloses at least one common phase-coupling element in a plan view. 18.The method of claim 17, wherein each of the n loop circuits encloses allof the phase-coupling circuits in a plan view.
 19. The method of claim14, wherein the loop circuits constituted by the inverter circuits andthe wires do not include any of the phase-coupling elements, and eachthe n loop circuits constituted by the inverter circuits and the wiresencloses at least one common phase-coupling element in a plan view. 20.The method of claim 19, wherein each of the n loop circuits encloses allof the phase-coupling circuits in a plan view.